As recognized in the art, opal glasses contain small particles which scatter light passing through the glass, thereby rendering such glasses light diffusing. Hence, opal glasses will always consist of a transparent glassy matrix and at least one other phase dispersed therewithin. The dispersed phase(s) may be either crystalline or amorphous. The primary characteristics of the dispersed or opacifying phase which determine the density of light transmission include the refractive index, the dispersion, the size and shape of the particles, the particle distribution, and the absolute number of particles.
There are two broad classes of opal glasses, viz., spontaneous opals and thermally opacifiable or reheat opals. Spontaneous opal glasses are characterized by the fact that the light-diffusing phase separates out ("strikes in") during the cooling and forming of the melt into a glass article. In contrast, the rate of opal development, i.e., the rate at which the light-diffusing phase separates out of the glassy matrix, is relatively slow in the thermally opacifiable glasses. Consequently, upon cooling and shaping the melt into a glass article, a substantially clear or only faintly opacified appearance is observed. The glass article must be reheated to temperatures in and/or above the transformation range of the glass to promote separation of the opacifying phase(s). It will be appreciated that, from a commercial point of view, spontaneous opal glasses are much to be preferred since no reheating is demanded to achieve the desired opacity.
There are two general categories of spontaneous opal glasses, the first being characterized as having an amorphous (non-crystalline) opacifying phase and the second having a crystalline opacifying phase. The first type has been termed immiscible opals, i.e., opals wherein the opacifying phase is a glass which is not soluble in the matrix. The most ommon immiscible opals contain borate or phosphate in the opacifying phase. U.S. Pat. Nos. 2,559,805 and 3,275,492 are illustrative of those. U.S. Pat. No. 3,661,601 describes another immiscible opal containing phase separated droplets or opacifying particles consisting of CaO and F or CaO, F, B.sub.2 O.sub.3, and SiO.sub.2. Numerous crystals have been precipitated to constitute the opacifying phase, the most common being either an alkali metal fluoride (most frequently NaF) or an alkaline earth metal fluoride (most often CaF.sub.2).
In summary, a spontaneous opal glass attains the vast majority of its opacity during the cooling of the melt to a glass article and requires no reheating. Thus, the opacifying agent strikes in during the shaping of the melt to a glass article utilizing such conventional glass forming techniques as blowing, casting, drawing, pressing, rolling, and spinning.